1. Field of the Invention
The present invention relates to a straight traveling hydraulic circuit for heavy construction equipment using a plurality of hydraulic pumps, which can improve the traveling manipulability of the equipment during a combined operation in which a traveling operation and a working operation are simultaneously performed.
More particularly, the present invention relates to a straight traveling hydraulic circuit, which, in case of performing a combined operation in which a fine operation of a working device and a fine traveling of a traveling device are simultaneously required, such as burying of waste pipes under a road in a downtown, can allow heavy construction equipment to travel independent of the working device, thereby preventing a declination or sudden traveling of the equipment.
2. Description of the Prior Art
Generally, heavy construction equipment, such as excavators, are constructed to operate actuators (traveling motors) of a traveling device and actuators (boom cylinder, arm cylinder, bucket cylinder and the like) of a working device using a plurality of hydraulic pumps. In case of performing a combined operation in which a traveling operation and a working operation are simultaneously performed, a hydraulic circuit includes a straight traveling valve installed on an upstream side of one hydraulic pump. The hydraulic circuit shifts the straight traveling valve in response to an operation signal of the traveling and working devices.
Referring to FIG. 1, a conventional straight traveling hydraulic circuit includes a plurality of first and second hydraulic pumps 1 and 2, and a pilot pump 3; a first traveling control valve 9 for a left traveling motor 8 and control valves 10 to 13 for a working device (swing, boom, option device, and arm) which are installed in a first center bypass passage 20 of the first hydraulic pump 1; a second traveling control valve 14 for a right traveling motor 8a and control valves 15 to 17 for the working device (boom, bucket, and arm) which are installed in a second center bypass passage 22 of the second hydraulic pump 2; and a straight traveling valve 4 installed on an upstream side of the second center bypass passage 22 and shifted to supply a hydraulic fluid fed from the first hydraulic pump 1 to the first and second traveling control valves 9 and 14 and to supply the hydraulic fluid fed from the second hydraulic pump 2 to the control valves 10-13 and 15-17 for the working devices, in response to a pilot signal pressure applied from the pilot pump 3.
Reference numerals 6 and 7a denote check valves, and 7 denotes a fixed orifice.
The left traveling motor 8 and the working device are driven by the hydraulic fluid fed from the first hydraulic pump 1, while the right traveling motor 8a and the working device are driven by the hydraulic fluid fed from the second hydraulic pump 2.
In case that the straight traveling valve 4 is shifted by the pilot signal pressure from the pilot pump 3, a part of the hydraulic fluid fed from the first hydraulic pump 1 is supplied to the first traveling control valve 9 via the first center bypass passage 20. At the same time, a part of the hydraulic fluid fed from the first hydraulic pump 1 is supplied to the second traveling control valve 14 via a passage 21 and the straight traveling valve 4.
Meanwhile, a part of the hydraulic fluid fed from the second hydraulic pump 2 is supplied to the control valves 15-17 for the right working device through a passage 23, and simultaneously, a part of the hydraulic fluid fed from the second hydraulic pump 2 is supplied to the control valves 10-13 for the left working device via the shifted straight traveling valve 4 and a passage 24.
That is, the hydraulic fluid fed from the first hydraulic pump 1 serves to operate the traveling device, and the hydraulic fluid fed from the second hydraulic pump 2 serves to operate the left and right working devices.
With the structure of the conventional hydraulic circuit, the pressure corresponding to the high load of the left working device which is generated in the passage 24 and the pressure corresponding to the high load of the right working device which is generated in the passage 23 are supplied to the second traveling control valve 14 via a fixed orifice 7 and a check valve, when the straight traveling valve 4 is shifted to carry out the combined operation in which the traveling device and the working device are simultaneously driven.
As a result, the left and right traveling pressure is not balanced. This results in a failure in keeping the heavy construction equipment travel straight and thereby creating a left or right declination of the equipment. Also, a shock such as a sudden traveling happens due to the sudden supply of the hydraulic fluid to the traveling device.
In addition, if the traveling side pressure is relatively high while the working side pressure is relatively low, the hydraulic fluid is fed to the working device, so that the traveling is decelerated. This results in reduction of the operability of the traveling device and the working efficiency.
More specifically, in case of burying heavy pipes, such as large waste pipes, under a road in a downtown, the straight traveling valve 4 is shifted, and then the high load working is performed by manipulating operation levers of the first and second traveling control valves 9 and 14 not in a full stroke but in a half stroke (so-called “half lever”) and also manipulating operation levers in a half lever.
In this instance, when the operation lever of the control valves 10 to 13 for the left working device and the operation lever of the control valves 15 to 17 for the right working device are manipulated not in a full stroke, but in a half stroke (so-called “half lever”), the first and second center bypass passages 20 and 22 are not fully closed, but opened in half.
As a result, since the hydraulic fluid of the first hydraulic pump 1 serves to operate the traveling device, and the hydraulic fluid of the second hydraulic pump 2 serves to operate the left and right working devices, the roles are not fully shared.
If the first traveling control valve 9 is shifted by a half due to the half lever, the hydraulic fluid fed from the first hydraulic pump 1 is supplied to the center bypass passage of the control valves 10 to 13 for the downstream side working device via the first center bypass passage 20 and the first traveling control valve 9.
In this instance, the pressure applied to the first hydraulic pump 1 is varied depending upon not the pure traveling pressure but the stroke of the control valves 10 to 13 for the downstream working device.
The flow passage of hydraulic fluid which is fed from the first hydraulic pump 1 and then flow through the first center bypass passage 20 and a pressurized tank 18 is interrupted by the spool operation of the control valves 10 to 13 for the downstream working device. As a result, the delivery pressure of the first hydraulic pump 1 is increased, and thus the traveling side pressure applied to the first hydraulic pump 1 is increased.
It is similarly applied to the second traveling control valve 14 and the control valves 15 to 17 for the downstream working device.
Accordingly, as the load pressure generated by the working device is increased, the pressure corresponding to the high load of the working device is transferred to the second traveling control valve 14 via the fixed orifice 7 and the check valve, thereby increase the pressure in the right traveling motor 8a. 
As a result, the straight traveling is not ensured dependent of the working device, and the traveling device is supplied with the pressure according to the shift of the control valve for the working device. Therefore, the left and right traveling pressure is not balanced, thereby creating left or right declination or sudden traveling of the equipment and thus reducing the operability of the traveling device.
In addition, if the traveling side pressure is high and the working device side pressure is low, the hydraulic fluid travels to the working device, thereby decreasing the working efficiency.